The present invention relates to guidance systems for air-to-air missiles equipped with infrared seeking sensors or radar systems.
The operation of conventional air-to-air missile guidance systems used in missiles with infrared seeking sensors or radars is well known. Most of these missiles operate in a "fire and forget mode". At the time of launching of a missile from an aircraft the target is brought into the field of view of the sensor, or radar antenna, of the missile. Once the missile is launched it uses its self-guidance system to track and then home in on the target by maintaining the target at the center of its field of view by continually updating the missile's flight path.
For the specific example of missile's equipped with infrared seeking sensors the process of updating the missile's flight path is as follows. At the time of launching the sensor is directed substantially towards the target so that an infrared radiating "hot" spot of the target is located at, or near, the center of its field of view. As the target moves away from the center of the field of view of the missile's sensor so that the missile's flight path correspondingly moves off target, the sensor rotates independently of the missile's body to bring the target's infrared radiating hot spot back into the center of its field of view. A signal representative of the spatial rotation angle through which the sensor rotated during this manoeuvre is transmitted to a control unit which in turn operates the missile's steering system which, by way of a non-limiting example, activates the missile's fins to re-align the missile thereby ensuring that its flight path is again on target. This procedure of rotation of the missile's sensor and re-aligning of the missile, has to be performed continuously, or quasi-continuously, since a missile cannot make sudden changes in direction, i.e., its flight path is always smooth, even though the missile's sensor is fitted on gimbals that allow for fairly large angles of rotation.
Missiles fitted with sensors that are capable of rotating independently of the missile and therefore "seeing" targets that are off boresight are termed "off-boresight missiles". The angle through which the seeker rotates from boresight is termed the "off-boresight angle". For a review of the properties of various high performance short-range off-boresight missiles see: Aviation Week and Space Technology, pp. 36-49, Oct. 16, 1995, the field of view of the sensor is relatively small (about 3.degree.). Hence, in order for the missile not to completely "lose sight" of the target, the updating of the missile's flight path has to be continuously performed. The process involved in updating an air-to-air missile equipped with a radar system is similar, the main difference being that in this case the target is maintained at the center of the field of view of the radar's antenna by maintaining a maximum target echo as received by the radar system.
There are, therefore, clearly a number of serious drawbacks with conventional fire and forget missiles. For a start, the missile has to be in a "seek mode" from the moment of launching. Hence, for such missiles the target cannot be outside the field of view of the missile either at the time of launching of the missile, or at any time after launching. This means that targets outside of the field of view of the missile, e.g., behind or on a side of the aircraft (i.e., at an angle of greater than 90.degree. from boresight), on which the missile is mounted, cannot be acquired by the missile at the time of launching. Another drawback of such missiles is that should the target escape completely from the missile's field of view after launch, there is no way to set the missile back on a homing flight path toward the target. Another well known drawback is the susceptibility of these missiles to counter measures, which normally take the form of flares for infrared seeking missiles and chaff for missile's equipped with radars. In both of these cases the counter measures act as decoys which, as far as the missiles are concerned, are valid targets.
Although future missile systems are planned to include various Counter Counter Measures (CCM), e.g. CCM's which utilize micro processors for comparing various characteristics of the decoy with those of the target (e.g., for the infra red sensor case these characteristics could be, the spectrum, intensity and velocity of the radiation emitted by a flare and by the exhaust of the target), they would require mounting an appropriate sub-system on a missile. Needles to say, that existing missiles would have to be fitted with such a sub-system in order to enjoy decoy counter counter-measure capability.
A further and well known problem of off-boresight missiles is that if in the pursuit of a target they do make a sudden large angled turn (e.g., just after launch) they could well lock on to a friendly aircraft. The ability to distinguish between friendly aircraft and enemy targets (whether aircraft or missiles) so as to avoid friendly fire situations is would clearly a desirable feature of such missiles.
Even in apparently favorable situations wherein the target is in the field of view of the missile's sensor the infrared signal reaching the missile from the target may be very weak. Such a situation could arise when, for example, the target is approaching the missile so that the target's hot spot (at its rear) is effectively hidden from the sensor's field of view.
Therefore a need exists for missile guidance systems that can track and home in on a target situated outside the field of view of the missile either at the time of launching of the missile, or at any time after launching. Another need exists for missile guidance systems that can track and home in on a target even in the presence of decoy counter measures, or when the signal from the target that is detected by the sensor is very weak. Preferably, the proposed missile guidance system should inherently incorporate in it counter counter-measure capability, without the necessity of an additional CCM sub-system.